Showing papers in "Applied Catalysis A-general in 2019"

Passive NOx adsorber: An overview of catalyst performance and reaction chemistry

Abstract: Passive NOx adsorbers have been proposed to help address cold start NOx emissions. Although the original concept was proposed earlier, it is not until the last few years that more significant efforts focused on understanding the fundamentals of this technology. Several groups of potential PNAs have been evaluated for low temperature NOx storage and subsequent release, and the laboratory-scale experimental testing results appears to be promising. This review will summarize the currently reported materials for PNA application, their unique behaviors when operating under different conditions, systematically evaluate and compare their performances and discuss the relevant reaction chemistry. The discussion around each specific topic is based on literature reported experimental data from testing and characterizing potential PNA candidates and will put more emphasis on the recent advances in the fundamental understanding of this technology. Since at this stage the mechanism of the adsorption and desorption processes is not fully understood, performance evaluation and reaction chemistry will be discussed independently. Unresolved issues are listed at the end of each section as an attempt to summarize the critical unknowns within a specific research topic.

Reduced perovskite LaNiO3 catalysts modified with Co and Mn for low coke formation in dry reforming of methane

Abstract: In dry reforming of methane (DRM), coke deposition on the Ni-based catalyst is the main cause of instability of the process. Perovskite LaNiO3 is a well-known highly active catalyst precursor for DRM, but the La-Ni catalyst derived from it is susceptible to severe coke deposition and thus difficult for practical applications. To improve its stability and activity, Co and Mn are introduced to develop a tri-metallic LaNi0.34Co0.33Mn0.33O3 catalyst precursor. The role of Mn is to improve the stability of the catalyst, whereas Co is an additional active component to increase the reaction rates. A strong metal and support interaction mediated by MnO is noted in the tri-metallic catalyst, which contributes to a synergistic effect of the tri-metals to sustain the high activity and stability under the harsh conditions of DRM.

Metal–organic frameworks: A tunable platform to access single-site heterogeneous catalysts

Abstract: Heterogeneous catalysts streamline industrial processes due to high stability and ease of separation. Efforts toward predictive catalytic activity and rational design of next-generation systems are impeded by structural ambiguity stemming from the presence of multiple irregular active sites within a catalyst. The uniformity of metal–organic frameworks (MOFs) provides a platform on which to develop single site catalysts in order to delineate atomically precise design rules. In this critical review, we highlight the development and progress of MOF-based single site catalysts and provide an outlook for further advancement of the field.

Palladium/Beta zeolite passive NOx adsorbers (PNA): Clarification of PNA chemistry and the effects of CO and zeolite crystallite size on PNA performance

Abstract: Model 0.3 wt% and 1 wt% Pd/Beta was synthesized with nano-sized Beta crystals (average size O2 in dry streams promotes NO storage via additional formation of nitrosyl (NO+) ions; it also leads to stabilization of significant amounts of N2O3 in BEA pores. In the presence of water, NO storage capacity is dramatically lowered. However, in the presence of CO (which is always present in exhaust gas) the performance improves. With the aid of spectroscopy and PNA measurements we show that in the presence of CO, stable mixed palladium(II) carbonyl nitrosyl complex [Pd(II)(CO)(NO)] is formed and responsible for enhanced NOx storage. This points to a general conclusion that the species responsible for NOx storage on various zeolites in the presence of water and CO, are in fact, Pd(II)(CO)(NO). Decreasing the Pd loading to 0.3 wt% improves Pd dispersion and NO/Pd storage ratio. We also demonstrate that varying crystal sizes of BEA leads to significant changes in the NOx release temperature as well as NO/Pd ratio. Larger, defect free, more hydrophobic BEA crystals store more NOx and release it at higher temperature. Furthermore, larger, defect free and more hydrophobic BEA crystals stay significantly more active for PNA after hydrothermal aging, losing little activity compared with defective nanocrystals. High-field 27Al NMR results show that larger, hydrophobic BEA crystals are more resistant to de-alumination upon hydrothermal aging (HTA), and thus are more attractive for PNA storage with Pd.

On the deactivation of Ni-Al catalysts in CO2 methanation

Abstract: This work provides detailed knowledge of long-term deactivation of Ni catalysts in CO2 methanation. NiAlOx mixed oxides with varying Ni loading as well as a 17 wt-% Ni/γ-Al2O3 catalyst were synthesized via co-precipitation and incipient wetness impregnation, respectively. The catalysts were aged at 523, 573 and 623 K under equilibrium conditions up to 165 h. Periodic activity measurements under differential conditions reveal severe deactivation. The stability of co-precipitated systems increases with decreasing Ni content on the expense of catalyst activity. Ni/γ-Al2O3 exhibits a lower stability as a comparable mixed oxide. A power law model is applied for the kinetic description of deactivation. Catalyst samples are characterized by means of temperature programmed desorption of H2 (H2-TPD) and CO2 (CO2-TPD), pulsed H2 chemisorption, XRD, FT-IR spectroscopy, XPS and N2 physisorption. Main deactivation mechanisms in the co-precipitated samples are found to be Ni particle sintering, a loss of BET surface area as well as a reduction of CO2 adsorption capacity and medium basic sites, along with structural changes of the mixed oxide phase. Ni particle growth and a decrease in BET surface area lead to deactivation of the impregnated sample. Structure-activity correlations imply a complex interplay of governing deactivation phenomena as well as structure sensitivity.

Research progress, challenges and perspectives on the sulfur and water resistance of catalysts for low temperature selective catalytic reduction of NOx by NH3

Abstract: Low temperature selective catalytic reduction (SCR) is an efficient and promising technology to transform NOx into N2 from flue gases, which has broad application prospects. However, the vulnerability of low temperature DeNOx catalysts to poisoning by SO2 and H2O restricts their industrial application. Therefore, it is vitally important to develop new low temperature DeNOx catalysts that resist poisoning. This paper reviews the mechanisms of SO2 & H2O poisoning of low temperature SCR catalysts, the strategies for improving the resistance of low temperature SCR catalysts to SO2 & H2O poisoning, application of theoretical calculations in the development of SO2 & H2O resistant low temperature SCR catalysts as well as novel ideas for developing poisoning resistant low temperature SCR catalysts. Moreover, the studies dealing with the regeneration of low temperature DeNOx catalysts are also included. Based on the problems detailed in this review, the challenges and the emphasis for future research in this area are also proposed. We believe that this review will provide an excellent foundation and good guidance for the future design and the development of improved SO2 & H2O-tolerant low temperature DeNOx catalysts.

CdS nanorods anchored with CoS2 nanoparticles for enhanced photocatalytic hydrogen production

Abstract: Herein, we report the use of cobalt sulfide (CoS2) as efficient and inexpensive co-catalyst of CdS nanorods for photocatalytic water splitting. The aim is to explore the use of earth-abundant cobalt species to replace precious metals for the photocatalytic reactions. The results of first-principles DFT simulation and planar-averaged differential charge density calculation reveal that at the CoS2/CdS interface, CoS2 has zero band gap which is a class nature of precious metals, and functions as electron trap to enhance the transfer of hot electrons from CdS to CoS2. Owing to the merits of efficient charge separation, high exposure of active sites as well as large surface area, the CoS2/CdS composites exhibit outstanding photocatalytic activity in H2 production under visible light (˜58 mmol·g−1 h−1), which is about 19 times that of CdS nanorods alone and 3 times that of 1 wt%Pt/CdS under the same conditions.

Critical review on the active site structure of sulfated zirconia catalysts and prospects in fuel production

Abstract: This article critically reviews the literature on sulfated zirconia catalysts for variety of important hydrocarbon and oxygenate reactions including ethylene dimerization, isomerization, alkylation and esterification. The catalyst synthesis method-resulting molecular structure relationships, stability and reactive transformations of the active site in response to gas phase environment, reaction mechanisms, reaction intermediates, the corresponding kinetics and effect of some popular promoters are covered. Key structural features of the active sites have been extensively investigated but the conclusive structure is yet to be determined due to the diversity of the preparation methods (nature and crystallinity of the support, sulfate deposition method, sulfate coverage) as well as the diversity of the experimental conditions used during the spectroscopic determination. Similarly, the effect of the promoters and the synergistic effects of the promoter/sulfate sites in the presence of hydrocarbons are not well-defined. Finally, multiple theories exist for the reactive mechanisms on sulfated zirconia but most rely on the homogeneous reaction mechanisms, such as those for ethylene dimerization and butane alkylation with butane. Collectively, these show the lack of a fundamental understanding and the need of modern catalysis studies of this important catalytic system.

Catalytic hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to biofuel 2, 5-dimethylfuran

Abstract: 2, 5-dimethylfuran (DMF), which is synthesized through catalytic hydrogenolysis of 5-hydroxymethylfurfural (HMF), is one of the most promising liquid biofuels for transportation because of its high energy density, high boiling point, and water insolubility. In recent years, studies of the hydrogenolysis of HMF to DMF have attracted increasing attention, with numerous novel catalytic systems and significant research achievements being reported as a result. Thus, in this review, we focus on hydrogenolysis of HMF into DMF through various catalytic systems, and systemically summarize the latest advances and progress on this reaction from the perspective of catalysts, including noble and non-noble metal-based catalysts. Moreover, the reaction mechanisms for different catalysts, the limitations and problems in current research, and potential research trends are discussed in this review.

Insight into the hydrogenation of pure and crude HMF to furan diols using Ru/C as catalyst

Abstract: 5-hydroxymethylfurfural (HMF) is one of the most important renewable platform-chemicals, a very valuable precursor for the synthesis of bio-fuels and bio-products. In this work, the hydrogenation of HMF to two furan diols, 2,5-bis(hydroxymethyl)furan (BHMF) and 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF), both promising renewable monomers, was investigated. Three commercial catalysts, Ru/C, Pd/C and Pt/C, were tested in the hydrogenation of aqueous HMF solutions (2–3 wt%), using a metal loading of 1 wt% respect to HMF content. By appropriate tuning of the process conditions, either BHMF or BHMTHF were obtained in good yields, and Ru/C resulted the best catalyst for this purpose, allowing us to obtain BHMF or BHMTHF yields up to 93.0 and 95.3 mol%, respectively. This catalyst was also tested for in the hydrogenation of a crude HMF-rich hydrolyzate, obtained by one-pot the dehydration of fructose. The influence of each component of this hydrolyzate on the hydrogenation efficiency was investigated, including unconverted fructose, rehydration acids and humins, in order to improve the yields towards each furan diol. Moreover, ICP-OES and TEM analysis showed that the catalyst was not subjected to important leaching and sintering phenomena, as further confirmed by catalyst recycling study.

Binary metal-organic frameworks: Catalysts for the efficient solvent-free CO2 fixation reaction via cyclic carbonates synthesis

Abstract: Two types of binary metal-organic frameworks (MOFs) using Cu (from Cu-BTC) and Zr (from UiO-66) as the metal centers have been synthesized by the solvothermal method and characterized using different physicochemical techniques. The catalytic potential of binary MOFs was clearly demonstrated for the cycloaddition reaction of carbon dioxide (CO2) with epoxides under solvent-free conditions. The effects of various parameters such as catalyst amount, temperature, reaction time, and CO2 pressure were studied, and a moderate set of reaction conditions (0.16 mol% of the catalyst, 60 °C, 8 h and 1.2 MPa CO2 pressure) was selected for detailed analysis. The synthesized Cu/Zr MOFs were used for the CO2-epoxide cycloaddition reaction with a tetrabutylammonium bromide (TBAB) co-catalyst. The UiO-66/Cu-BTC displayed excellent conversion of epichlorohydrin (ECH) with >99% selectivity. The appreciable conversion of ECH with the UiO-66/Cu-BTC/TBAB system was influenced by the synergistic effect of the Cu and Zr metals and the Br ion from TBAB. The scope of extending this catalysis to various epoxides was established, and a recyclability study was also conducted. Finally, based on our previous DFT (density functional theory) studies and experimental inferences, a plausible reaction mechanism for the binary MOF-catalyzed epoxide-CO2 cycloaddition reaction was proposed.

Optimizing Pd:Zn molar ratio in PdZn/CeO2 for CO2 hydrogenation to methanol

Abstract: We report the compositional optimization of Pd:Zn/CeO2 catalysts prepared via sol-gel chelatization for the hydrogenation of CO2 under mild reaction conditions. The formation of a PdZn alloy, which is the main active phase for this reaction, was maximized for the catalyst with a Pd to Zn ratio close to 1. For this catalyst, a maximum conversion of 14%, close to thermodynamic equilibrium, and high selectivity to methanol (95%) were achieved at 220 °C, 20 bar, 2400 h−1 GHSV and H2:CO2 stoichiometric ratio of 3:1. The formation of PdZn alloys was achieved by reducing the catalyst precursor at 550 °C under hydrogen flow and confirmed by XRD. XPS study confirmed the presence of Pd°, being maximum for the optimized catalyst composition. At lower temperature, i.e. 180 °C, 1.0PdZn catalyst showed 100% selectivity to methanol with 8% CO2 conversion. RWGS reaction is responsible for the production of CO and its selectivity increases with temperature. In situ DRIFTS suggests that CO2 is activated as adsorbed CO3- species over CeO2. Surface micro-kinetics demonstrates that methanol can be formed either via formaldehyde or formic acid surface intermediates.

Direct conversion of CO2 into methanol over promoted indium oxide-based catalysts

Abstract: Supported indium oxide catalysts are investigated for the CO2 hydrogenation to methanol at a total pressure of 40 bar (528–573 K) using a laboratory flow reactor. Surface reducibility, optical spectral characteristics, and catalytic rates and selectivity were correlated to catalyst composition. Promoted catalysts, especially Yttrium or Lanthanum-promoted indium oxide, require higher temperatures (H2-TPR) for surface reduction and display higher CO2 desorption temperatures (CO2-TPD). The promoted samples also have ˜20% higher methanol selectivity compared to the non-promoted catalyst, while having similar methanol formation rates (0.330–0.420 gMeOH gcat.−1 h−1 at 573 K). From 528 K to 558 K, methanol selectivity was over 80%, over all the promoted catalysts, and nearly 100% selectivity was observed at the low temperature range (˜528 K) investigated. The reaction kinetics of Y-promoted catalyst and the results of CO co-feeding experiments suggest that formate pathway is the likely reaction mechanism for methanol formation.

Correlation between the physicochemical properties and catalytic performances of micro/mesoporous CoCeOx mixed oxides for propane combustion

Abstract: A series of micro-mesoporous CoCeOx catalysts have been successfully prepared using a double template combining sol-gel method. The catalytic performance of novel CoCeOx catalysts are investigated and compared with pure Co3O4 and CeO2 catalysts for total oxidation of propane. It is found that the Co1Ce1 catalyst shows the highest catalytic activity (T50 = 217 °C) as well as a good reaction stability and water tolerance among the five catalysts. The Co3O4, CeO2 and CoCeOx catalysts are characterized using XRD, BET, Raman, XPS, H2-TPR, O2-TPD, HRTEM, HAADF-STEM and in situ DRIFTS. The results demonstrate that the larger BET surface area, smaller grain size, stronger reducibility and more active oxygen species of the Co1Ce1 catalyst are responsible for its outstanding catalytic performance among the five catalysts. Moreover, the synergistic effect between Co and Ce over CoCeOx catalysts are probably relevant to the formation of CoxCe1−xO2−σ solid solution. In addition, on the basis of the results of XPS, kinetic analysis and in situ DRIFTS, the surface Co3+ ions and active oxygen species are regarded as the major active sites of the CoCeOx catalysts for total oxidation of propane, and then a scheme of reaction model based on Langmuir-Hinshelwood mechanism is suggested at last. It can be expected that the micro-mesoporous CoCoOx catalysts are promising materials for VOCs removal, and the results in this research may also provide some new insights into the catalyst design and mechanism exploration for VOCs catalytic oxidation.

Effect of reduction treatments (H2 vs. CO) on the NO adsorption ability and the physicochemical properties of Pd/SSZ-13 passive NOx adsorber for cold start application

Abstract: Pd/SSZ-13, which has ability to adsorb NO at low temperature, is potentially used as a passive NOx adsorber for cold start application. We investigate the changes in the physicochemical properties and low temperature NO adsorption ability of Pd/SSZ-13 resulting from the reduction treatments with H2 or CO at various temperatures. Treatment with H2 at 500 °C does not give rise to the decrease in NO adsorption ability. However, the ability begins to decrease gradually with the increasing reduction temperature from 600 to 900 °C. On the other hand, CO treatment at 500 °C induces a significant decrease in NO adsorption ability. Combined EXAFS, HAADF-STEM and H2-TPR results clearly demonstrate that CO treatment induced more severe sintering of Pd species than H2 one at the same reduction temperature. After re-oxidation at 500 °C, H2-treated sample generated Pd ion species, which are the active site for NO adsorption at low temperature, more abundantly than CO-treated one, well corresponding to the higher NO adsorption ability over the former sample. Such phenomenon is attributed to the facile sintering behavior due to the high mobility of Pd-carbonyl complex upon the treatment with CO. It can be concluded that the reduction treatments with H2 and CO have different effect on Pd sintering, which is regarded as the main deactivation mechanism of Pd/SSZ-13 passive NOx adsorber.

Activated carbon supported bimetallic catalysts with combined catalytic effects for aromatic nitro compounds hydrogenation under mild conditions

Abstract: Non-noble nickel catalysts have been widely studied and tried in hydrogenation, however the problem of nickel particle sintering is more and more common in high-loaded nickel catalysts. A series of highly dispersed bimetallic Ni-M/AC (M = Cu, Co, Fe or Zn) catalysts were prepared by incipient wetness impregnation methods and applied in 1-nitronaphthalene hydrogenation to 1-naphthylamine under mild reaction conditions. The prepared catalysts were characterized by XRD, BET, H2-TPR, TEM, HRTEM, HAADF-STEM, XPS, ICP, FT-IR and H2 chemisorption. The results show that the introduction of the metal promoter inhibits the sintering of the nickel and enhances the reducibility of the catalysts, leading to higher ratio of effective Ni° on the surface of the support, especially for Ni-Zn/AC sample. Moreover, the results of XPS indicate that the electron donating effect of Cu promoter increases surface electronic density of Ni, as a result, the electron-rich Ni might be produced because of the interfacial electronic effect, which favors the desorption and further impedes the hydrogenation of N-naphthylhydroxylamine. Ni-Zn/AC-350 with smaller nickel particles, better dispersion and larger content of effective Ni° presents the best catalytic performance in 1-nitronaphthalene hydrogenation to 1-naphthylamine under mild reaction conditions, it gives 100% conversion of 1-nitronaphthalene and 96.82% selectivity to 1-naphthylamine under 0.6 MPa and 90℃ for 5 h. Additionally, superior performances are also obtained in hydrogenation reactions of nitrobenzene, chloronitrobenzene, 1,5-dinitronaphthalene and 1,8-dinitronaphthalene over Ni-Zn/AC catalysts. With good hydrogenation activity the catalyst shows, the application prospect in industrial production of aromatic amine from aromatic nitro compounds has been becoming more and more extension.

A highly active and selective mesostructured Cu/AlCeO catalyst for CO2 hydrogenation to methanol

Abstract: Mesostructured catalysts with optimized microstructure and surface properties endowed them attractive catalytic performance. Herein, the mesostructured AlCeO supports with different weight ratio of Al2O3 and CeO2 have been prepared on the basis of a self-assembly sol-gel process, then 15 wt.% Cu was loaded onto the mesostructured AlCeO supports by incipient wetness impregnation method. The mesostructured AlCeO supports with varied ratio of Al2O3 to CeO2 resulted in the very different Cu particle size, and for the optimized catalyst Cu/AlCeO-7 (the amount of Al2O3 in AlCeO support was 70 wt.%), the Cu crystal size was around 7 nm based on the characterization of XRD and TEM. The Cu/AlCeO-7 catalyst had strong metal-support interactions (SMSI) and more Cu-ceria interface, which was confirmed by the XPS. Furthermore, the Cu/AlCeO-7 catalyst showed the highest exposed Cu surface area (SCu) (25.5 m2/gcat) from the analysis results of H2-TPR and dissociative chemisorption of nitrous oxide. The CO2-TPD results demonstrated the Cu/AlCeO-7 had strong surface basicity. Hence, Cu/AlCeO-7 catalyst showed the highest space time yield of methanol (STYMeOH). The CO2 conversion (Cu/AlCeO-7, V(H2)/V(CO2) = 3/1, GHSV = 6000 mL h−1 g−1 and P = 4 MPa) was up to 22.5% at 553 K, the methanol selectivity can reach 94% at 473 K, and the STYMeOH was up to 7.2 mmol gcat.-1 h-1 at 493 K. The as-prepared mesostructured Cu/AlCeO-7 exhibited a promising catalyst candidate for CO2 hydrogenation to methanol.

SO2 promoted V2O5-MoO3/TiO2 catalyst for NH3-SCR of NOx at low temperatures

Abstract: V2O5-MoO3/TiO2 and V2O5-WO3/TiO2 monolithic catalysts were evaluated for selective catalytic reduction (SCR) of NOx with NH3 in the presence of SO2 and H2O. After a durability test at 220 °C for 30 h, the NOx conversion of V2O5-MoO3/TiO2 increased from 70% to 88% while it decreased slightly over V2O5-WO3/TiO2. The catalysts were characterized by H2 temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of NH3, thermo-gravimetric (TG) analysis, temperature-programmed decomposition (TPD) of deposits, temperature-programmed surface reaction (TPSR) of ammonium bisulfates with NO, surface-enhanced Raman spectroscopy (SERS), ultraviolet-visible (UV–vis) and diffuse reflection infrared Fourier transform (DRIFT) spectroscopy. There were fewer sulfates deposits on VMoTi-S catalyst due to lower SO2 oxidation which was related to less V O V bonds on the Mo-modified catalyst. The as-formed high proportioned stable tridentate sulfates on VMoTi-S catalyst acted as strong Bronsted acid sites, and more active polymeric vanadates remained on this catalyst to activate the absorbed NH4+. The separation of acid sites and active sites which facilitates NH2 formation was suggested to be responsible for the promoted SCR activity of V2O5-MoO3/TiO2 during NH3-SCR reaction.

Highly selective conversion of CO2 to light olefins via Fischer-Tropsch synthesis over stable layered K–Fe–Ti catalysts

Abstract: K–Fe–Ti layered metal oxides (LMO) were prepared using solid-state reaction followed by in situ reduction and applied in CO2 hydrogenation to light olefins via Fischer-Tropsch synthesis process. The results showed that the molar ratio of K/Fe/Ti had dramatic effects on the textural and structural properties of the LMO and the catalytic performance. Layered structure K–Fe–Ti exhibited high olefin selectivity and stability for CO2 hydrogenation. The light olefin selectivity reached approximately 60% with an olefin/paraffin value of 7.3 over 0.8K–2.4Fe–1.3Ti, and the exfoliated LMO through acid treatment was found to weaken the interaction between Fe and Ti. This enabled the reduction and activation of iron oxides easier to form iron carbide species and promoted a shift from the reverse water gas shift reaction regime to hydrocarbon synthesis regime, contributing to higher hydrocarbons while lower CO.

Highly-selective CO2 conversion via reverse water gas shift reaction over the 0.5wt% Ru-promoted Cu/ZnO/Al2O3 catalyst

Abstract: The reverse water gas shift (RWGS) reaction can be potentially used to convert captured CO2 into renewable synthetic fuels via syngas generation. However, this conversion pathway has not been yet realized because of a number of technological challenges, including a lack of catalysts which possess satisfactory catalytic performance. In this paper, the RWGS reaction over a 0.5 wt% Ru-promoted 40 wt% Cu/ZnO/Al2O3 catalyst is studied. Due to the Ru addition, CO2 conversion was improved more than two-fold as compared to the baseline Cu/ZnO/Al2O3 catalyst. The catalyst stability was significantly improved as well. Although Ru is known for its high activity in methanation reactions, the 0.5 wt% Ru-Cu/ZnO/Al2O3 catalyst maintained complete selectivity to CO formation. In order to investigate this intriguing finding, the catalyst was studied by XRD, SEM-EDS, STEM-EDS, and TPR, revealing the possibility of the formation of Ru-Cu core-shell nanoparticles. Further investigation have shown that the Ru-support interaction is also a crucial factor affecting the catalyst selectivity. To extrapolate the experimentally measured data, a rate expression was derived and kinetic parameters were estimated. The resulting reaction rate expression was implemented to assess the catalyst performance under a wider range of operating conditions.

Supported ultralow loading Pt catalysts with high H2O-, CO2-, and SO2-resistance for acetone removal

Abstract: Volatile organic compounds (VOCs) cause damage to atmospheric environment and human health. Supported noble metal catalysts are widely used to control VOCs emissions. The high cost, and low H2O-, CO2-, and SO2-resistance of such catalysts are worthy to be improved. Herein we fabricated Ce-, V-, or W-doped TiO2 supported ultralow loading Pt catalysts via the in-situ molten salt method, and evaluated their catalytic performance for acetone (major pollutants in pharmaceutical industry) removal. Under the present reaction conditions, all the catalysts exhibited high catalytic activity and stability for acetone oxidation, with the temperature required 90% acetone conversion being of 245 °C over 0.57 wt% CeO2-0.05 wt% Pt/TiO2. The doping of Ce, V, or W enhanced the H2O-, CO2-, and SO2-tolerance ability of 0.05 wt% Pt/TiO2. More than 85 %85%, 70%, or 60% of acetone could be removed even in the presence of 20 vol% water vapor, 10 vol% CO2, or 100 ppm SO2, respectively. The improvement in SO2-tolerance ability was due to the inhibition of SO2 adsorption and oxidation activity as well as Ti(SO4)2 or TiOSO4 formation. Acetone complete oxidation over 0.57 wt% CeO2-0.05 wt% Pt/TiO2 would follow the pathway: adsorbed acetone molecules → acetic acid and formic acid → carbonate species → CO2 and H2O. The present supported Pt catalysts might be suitable for oxygenated VOCs removal.

Influence of Ca/P ratio on the catalytic performance of Ni/hydroxyapatite samples in dry reforming of methane

Abstract: A series of Ni/hydroxyapatite samples presenting different Ca/P molar ratios were synthesised to study the influence of the hydroxyapatite support composition on their catalytic properties in the dry reforming of methane. Our results reveal that a preparation starting from a sub-stoichiometric composition (Ca/P The characterisation of the investigated Ni/HAP materials shows that their texture, surface chemistry (acid/base) and the Ni species distribution are mainly derived from the structural properties of the used support. The activity of the Ni/HAP samples in DRM shows that their performances follow this trend: Ni/HAP-D2 (Ca/P = 1.62) > Ni/HAP-D1 (1.57) > Ni/HAP-S (1.67) > Ni/HAP-E (1.73). The superiority of the sample with a Ca/P molar ratio of 1.62 was explained by a suitable surface chemistry consisting of an abundance of strong acid sites and basic sites. While the former act as anchoring sites for Ni species the latter serve as CO2 chemisorption sites producing intermediate species which in turn react with deposited carbon to form CO. This distribution together with its improved textural properties lead to the deposition of highly dispersed, efficient and coke resistant Ni species.

Nitrogen-doped carbon materials as a promising platform toward the efficient catalysis for hydrogen generation

Abstract: Nitrogen-doped carbon materials have emerged as versatile candidates with outstanding features as both support of metal-based catalysts and metal-free catalytic systems. Their use has been traditionally linked to electrochemical applications, but the number of applications in which these materials show their virtues has been exponentially increasing in the last decade. Particularly, catalysts based on metal nanoparticles supported on nitrogen-doped carbon materials have shown to be attractive for important catalytic reactions as they display enhanced performances endowed by well-dispersed, stabilized and electronically promoted nanoparticles as well as by the modified acid-base properties of the support. The present review is focused on the most recent breakthroughs achieved by these catalytic systems in the hydrogen production from some of the most representative hydrogen carrier molecules by highlighting the effect of the incorporation of nitrogen in the support matrix.

Dynamic CuII/CuI speciation in Cu-CHA catalysts by in situ Diffuse Reflectance UV–vis-NIR spectroscopy

Abstract: Copper exchanged CHA zeolites were investigated by in situ Diffuse Reflectance UV–vis-NIR spectroscopy in order to study the evolution of the copper environment in respect to the sample composition (2 samples differing in Si/Al and Cu content) and the activation procedure (in O2 or N2 from room temperature to 400 °C). In situ measurements allowed showing the different behavior of the two samples, both in the d-d and Ligand to Metal Charge Transfer regions, confirming a strong effect of the lattice composition to determine the copper environment and its reactivity. One of the two sample (Si/Al = 15 and Cu/Al = 0.5) was further investigated, in order to clarify the nature of the Cu-oxo species, formed along activation in oxygen. To shed light on this point, in situ measurements starting from a reduced form of the sample, obtained by activation in inert atmosphere at high temperature, were also considered. Additional suggestions on the nature of these species were obtained by resonant Raman measurements conducted in controlled atmosphere.

Enhanced CO2 hydrogenation to methanol over TiO2 nanotubes-supported CuO-ZnO-CeO2 catalyst

Abstract: TiO2 nanotubes (TNTs) have been extensively applied in many fields owing to their excellent porous structure and surface properties. Herein, a series of CuO-ZnO-CeO2/TNTs catalysts are synthesized through a deposition-precipitation method. The effects of TNTs content and support morphology on the catalyst physicochemical properties and catalytic performance are investigated. The incorporation of TNTs support into CuO-ZnO-CeO2 catalysts not only promotes CuO reducibility and improves the metallic Cu dispersion and specific surface area, but also enhances CO2 adsorption and increases the proportion of basic sites γ, thereby resulting in high CO2 conversion and CH3OH selectivity. It is also shown that CO2 conversion is positively correlated with the Cu specific surface area and that CH3OH selectivity is related to the proportion of basic sites γ. Due to the excellent reducibility, high metallic Cu surface area, superior CO2 adsorption and large proportion of basic sites γ, CuO-ZnO-CeO2/10 wt.% TNTs gives excellent catalytic performance and possesses a great potential as catalyst for methanol synthesis.

Controlling the acid-base properties of alumina for stable PtSn-based propane dehydrogenation catalysts

Abstract: The surface properties of catalyst supports are important in regulating the catalytic properties of heterogeneous catalysts. Herein, we studied the effect of acid-base properties of alumina on metal-support interaction and coke deposition, and investigated the stability of catalysts in propane dehydrogenation (PDH) using PtSn/Al2O3. We prepared γ-Al2O3 (A750) from ammonium aluminum carbonate hydroxide (AACH) and compared it with a commercial sample (Sasol Puralox SBA-200; P200). We loaded 0.5 wt% Pt and 0.9 wt% Sn on alumina then conducted propane dehydrogenation at 590 °C (WHSV = 5.2 h−1). PtSn/A750 and PtSn/P200 showed compatible initial activity (conversion = ˜50%) and selectivity (> 95%). After 20 h of reaction, PtSn/A750 showed a slight decrease in activity (39.9%) while the activity of PtSn/P200 dropped significantly (28.4%). Spent catalysts showed different metal sintering behavior and coke deposition which are well known causes for catalyst deactivation. A high strength of Lewis acid sites in A750 (higher Td in ethanol TPD) prevented the sintering of metal by strong metal-support interaction. Also, the lower number of Lewis acid sites in A750 than that of P200 reduced deposited coke on the catalysts (PtSn/A750: 1.8 wt% and PtSn/P200: 8.6 wt%). Furthermore, diffuse reflectance infrared Fourier-transform spectroscopy after CO adsorption at -150 °C clearly demonstrated that coke deposition was initiated from Lewis acid sites on the alumina surface, but then aromatization occurs at these sites. These results suggested that strong metal-support interactions to hold metal particles and less residual Lewis acid sites after metal loading to reduce coke deposition are important factors for designing stable and coke-resistant PtSn on alumina catalysts. Furthermore, precise characterization and understanding of the acid-base properties of alumina will contribute in developing catalysts with high stability.

Rh-Ni/MgAl2O4 catalyst for steam reforming of methane: Effect of Rh doping, calcination temperature and its application on metal monoliths

Abstract: The effect of adding small amounts Rh noble metal to 15 wt.% Ni/MgAl2O4 catalyst were studied for the steam reforming of methane. For this purpose, two series of catalysts were prepared and then the most active catalyst from this study was selected for metal monolith application. Both series of catalysts were tested at ambient and high pressures conditions. The first series of catalyst was used to determine the optimum Rh concentration by varying the Rh concentration from 0.1 to 1.0 wt.%. It was found that an optimum concentration of 0.5 wt.% Rh showed the highest activity and stability. The second series of catalysts containing 0.5 wt.%Rh-15 wt.% Ni/MgAl2O4 was used to study the effect of calcination temperature on the activity and stability. Studies on the second series of catalysts showed that the catalyst prepared by sequential impregnation and calcined at lower temperatures (600 °C after each Ni and Rh impregnation) showed the highest activity and stability. Among the several changes in factors detected due to Rh doping, the increase in catalytic activity for the catalyst calcined at lower temperatures was mainly attributed to its higher degree of reduction and higher abundance of Ni-Rh active sites. The most active Rh-Ni/MgAl2O4 catalyst was washcoated on FeCralloy metal monoliths and the performance compared with a packed bed and Ni/MgAl2O4 washcoated metal monolith. A significant increase of 24% in methane conversion was observed for Rh-Ni/MgAl2O4 washcoated metal monolith in comparison to NiMgAl2O4 washcoated metal monolith.

Robust Ni/Dendritic fibrous SBA-15 (Ni/DFSBA-15) for methane dry reforming: Effect of Ni loadings

Abstract: A series of spherical mesoporous Ni/Dendritic Fibrous SBA-15 (Ni/DFSBA-15) catalysts with different Ni loadings (3–15 wt.%) were successfully synthesized and catalytically investigated by methane dry reforming. The XRD and Raman analyses indicated the structural stability of DFSBA-15 regardless of the Ni loading quantity, and the NiO nanocrystalline size increased with increasing Ni loading. The BET analysis showed the surface area of the catalysts decreased upon increment in Ni loading. Meanwhile, the TEM images revealed the distribution of Ni particles over the spherical shape DFSBA-15, with the most homogeneous dispersion was shown by 10Ni/DFSBA15, while Ni agglomeration was observed for 15Ni/DFSBA-15. The optimal catalytic performance and stability was achieved by 10Ni/DFSBA-15 ( X C O 2 = 93.11%, X C H 4 = 91.76%, Y H 2 = 91.77%, YCO = 96.35%, SCO = 48.89%, S H 2 = 46.57% and H2/CO = 0.95) with no sign of deactivation was observed for 30 h time-on-stream, in agreement with the XRD, XPS, and TGA analyses of spent catalysts. The superior catalytic performance by 10Ni/DFSBA-15 could be credited to its strong Si-O-Ni interaction, moderate NiO crystallite size, and homogeneous active metal dispersion. The favorable properties of 10Ni/DFSBA-15 led to a strong synergistic effect between Ni active metal sites and the DFSBA-15 support and thus enhanced the reactivity between two gaseous reactants, CH4 and CO2. The 10 wt.% Ni loading was certified as optimal Ni content to sufficiently suppress the coke deposition on the DFSBA-15 and thus enhance the catalytic activity and stability.

The catalytic cracking of sterically challenging plastic feedstocks over high acid density Al-SBA-15 catalysts

Abstract: The catalytic cracking of polyolefinic waste materials over solid acid catalysts, such as zeolites, is a promising process for the production of useful fuels and chemicals. However, the inherent diffusional constraints of the microporous zeolites restrict the access of bulky polyolefin molecules to the active site, therefore limiting their effectiveness. To address this, a simple yet effective method of producing mesoporous Al-SBA-15 materials with a high density of Bronsted acid sites has been employed. These catalysts are shown to be very active for the catalytic cracking of low density polyethylene (LDPE), a common waste plastic. The acidic and textural properties of the catalysts were characterised by ICP-OES, XPS, XRD, N2 physisorption, propylamine-TPD, pyridine-FTIR and STEM and have been correlated with their catalytic activity. The product distribution from the catalytic cracking of LDPE has been shown to depend strongly on both the pore architecture and the Al content of the SBA-15 and thus the density and strength of Bronsted acid sites. Fine-tuning the Al content of the SBA-15 materials can direct the product distribution of the hydrocarbons. The Al-SBA-15 materials display increased cracking orientated towards aliphatic hydrocarbons compared to ZSM-5, attributed to the mesoporous nature of SBA-15, overcoming diffusional limitations.

Catalytic in-situ hydrogenation of 5-hydroxymethylfurfural to 2,5-dimethylfuran over Cu-based catalysts with methanol as a hydrogen donor

Abstract: A series of Cu-based catalysts with different supports were synthesized and studied for the in situ hydrogenation of 5-hydroxymethylfurfural (5-HMF) to dimethylfuran (DMF) using methanol as an economical hydrogen donor. The structures and properties of the four catalysts (Cu/Al2O3, Cu/ZnO, Cu/ZrO2, and Cu/CeO2) were characterized using X-ray diffraction (XRD), temperature-programmed reduction (H2-TPR), and temperature-programmed desorption of ammonia (NH3-TPD). The experimental results showed that the use of different supports for the Cu-based catalysts significantly influenced their activity for both H2 production from methanol and hydrogenation of 5-HMF. The catalyst Cu/Al2O3 showed the best catalytic activity, which can be attributed to the highest activity for the in situ H2 production from methanol, smallest Cu crystallite size, and strongest acidity. The effects of the substrate concentration, catalyst loading, and reaction temperature and time on the in situ hydrogenation of 5-HMF were systematically investigated to determine the optimum reaction conditions.